An innovative near-infrared-emitting chromenylium-cyanine probe decorated with benzothiazole and spiro-thiolactam units for the quantification of Hg²⁺ in real samples with ultra-trace sensitivity

The development of rapid, highly selective, and sensitive probes for the detection of highly toxic mercury (Hg²⁺) ions is of paramount importance for environmental and human health. Herein, we report the design and synthesis of a novel, near-infrared (NIR) emitting chromenylium-cyanine-based chemosensor (NIR20S) for the efficient detection of Hg²⁺ in real samples. To achieve target specificity, the amide moiety of the precursor was transformed into a thioamide group. This was achieved by exploiting the highly thiophilic nature of Hg²⁺. Upon the addition of Hg²⁺, the non-fluorescent and closed-spirolactam form of NIR20S undergoes a ring-opening process, triggering a robust ratiometric absorbance response and an intense turn-on NIR emission at 748 nm within 10 s. The specific binding mechanism and complexation of NIR20S with Hg²⁺ were comprehensively elucidated using several spectroscopic techniques, Job's plot and further supported by Density Functional Theory (DFT) calculations. The reversible probe exhibits exceptional selectivity over competing analytes across a broad pH range (2–9) with remarkable detection limits of 0.76 nM and 0.30 nM for UV-Vis and fluorescence measurements, respectively. The practical utility of NIR20S was successfully demonstrated through smartphone-assisted colorimetric analysis, silica-based test strips, and the quantification of Hg²⁺ in diverse environmental water samples and apple juice with high recovery rates. Furthermore, NIR20S was effectively employed for the bio-imaging of Hg²⁺ in living A549 cells, highlighting its robust potential for both on-site environmental monitoring and advanced biological applications.